1. Field of the Invention
The present invention relates to recording medium, and more particularly, to a patterned magnetic recording medium having reduced interaction between neighboring magnetic recording layers.
2. Description of the Related Art
In continuous magnetic recording media (hereinafter referred to as ‘continuous media’) in which a continuous magnetic film is used as a recording layer, the size of magnetic grains consisting the magnetic film should be small in order to increase the recording density of the continuous media. However, when the magnetic grain size is smaller than a critical value, a superparamagnetic effect occurs, and thus thermal stability of the magnetic grains is decreased, such that the data storage characteristics of the continuous media are deteriorated. Thus it is difficult to increase the recording density of the continuous media.
In order to overcome the limit of the recording density of the continuous media, patterned magnetic recording media (hereinafter referred to as ‘patterned media’) in which magnetic domains corresponding to bit regions are separated from each other have been suggested. The patterned media are disclosed in U.S. Patent Application No. US 2002/0068195 A1 and No. 2002/0154440 A1, and in Korean Patent Laid-Open Gazette No. 2005-0010338. The recording density of the patterned media is known to be 1000 Gbit/in2 or greater, which is significantly greater than that of the continuous media.
However, since magnetic interaction between neighboring magnetic domains is large in conventional patterned media, the switching field distribution is increased. Hereinafter, the above problem will be described in detail with reference to FIGS. 1A and 1B.
FIG. 1A is a cross-sectional view of a conventional patterned medium.
Referring to FIG. 1A, the conventional patterned medium includes a plurality of magnetic recording layers 100a through 100g (100) on a substrate 10. The magnetic recording layers 100a through 100g are disposed at a regular interval and are formed of a ferromagnetic material. Each magnetic recording layer is in the form of a pillar, and a non-magnetic boundary layer 150 is formed between the magnetic recording layers 100a through 100g. 
Each magnetic recording layer is a bit region in which data is recorded. Each magnetic recording layer is magnetized in a first direction D1 by a magnetic field generated by a recording head, or is magnetized in a second direction D2, which is opposite to the first direction D1. The magnetic recording layer magnetized in the first direction D1 and the magnetic recording layer magnetized in the second direction D2 may correspond respectively to a bit value of 0 (hereinafter, ‘0’) and a bit value of 1 (hereinafter, ‘1’). To record new data to the magnetic recording layer 100 which contains pre-recorded data, the magnetization direction of the magnetic recording layer 100 may need to be reversed. A magnetic field needed to reverse the magnetization direction is called a switching magnetic field.
Ideally, the absolute value of the switching magnetic field to record ‘0’ and absolute value of the switching magnetic field to record ‘1’ are equal, and the switching field distribution should be zero. However, in a conventional patterned medium, the switching magnetization distribution is greater than zero due to magnetic interaction between neighboring magnetic domains.
For example, in FIG. 1A where the magnetic recording layers 100a through 100g are all magnetized in the first direction D1, the absolute value of the switching magnetic field to reverse the magnetization direction of the magnetic recording layer 100d is smaller than the absolute value of the switching magnetic field needed to return the magnetization direction of the magnetic recording layer 100d to the first direction D1 again. The reason is that a magnetic field Hi, which is generated from magnetic recording layers 100a through 100c and 100e through 100g, which are located at the sides of the magnetic recording layer 100d, affects the magnetic recording layer 100d. In detail, when the magnetic recording layers 100a through 100c and 100e through 100g have the same magnetization direction (here, it is the first direction D1) as the magnetic recording layer 100d, the magnetic field Hi generated from the magnetic recording layers 100a through 100c and 100e through 100g and passing through the magnetic recording layer 100d has the second direction D2, which is opposite to the first direction D1. Thus the absolute value of the switching magnetic field to change the magnetization direction of the magnetic recording layer 100d from the first direction D1 to the second direction D2 is smaller than the absolute value of the switching magnetic field to change the magnetization direction of the magnetic recording layer 100d from the second direction D2 to the first direction D1.
FIG. 1B illustrates the hysteresis characteristic caused by a magnetic field H applied to the magnetic recording layer 100d. In FIG. 1B, M denotes the magnetization of the magnetic recording layer 100d. 
Referring to FIG. 1B, the hysteresis loop of the magnetic recording layer 100d is largely off-set to the right. Thus the difference between the absolute value of a switching magnetic field H1 for recording ‘0’ and the absolute value of a switching magnetic field H2 for recording ‘1’ is large.
The magnetization direction of the magnetic recording layers 100a through 100c and 100e through 100g may vary at different areas of a recording medium. Thus the difference between the absolute value of a switching magnetic field H1 for recording ‘0’ and the absolute value of a switching magnetic field H2 for recording ‘1’ also may vary at various areas of the recording medium.
The switching field distribution (%) is calculated by (ΔH/Hmin)×100, where ΔH denotes ∥H1|−|H2∥, and Hmin is the smaller value of |H1| and |H2|. When the magnetic anisotropic energy of the magnetic recording layer 100 is 2×106 erg/cm3, and 4 πMs is 1.0 Tesla, where Ms denotes saturation magnetization and one bit is switched by one time of application of a magnetic field, the switching magnetic distribution of a conventional patterned medium is 70%, which is significantly high.
Accordingly, it is difficult to secure recording reliability and data stability in conventional patterned media.